3 Answers
3

This is simply a unit that is cost efficient and can keep your
opponent at bay, making him think twice before moving out to attack
you.

For Terrans these units are: Banshees and Tanks

For Zergs these are: Infestors and Mutalisks

For Protoss these are: High Templars and Dark Templars

Why are they important and when do you use them?

These units should be used all the time because they are really
powerful, hence the name. These units give you time to do more things
without fearing a attack at every corner. these units, in other words,
control space and this is an important thing in all RTS games.

Once you have these units your opponent will focus on them in fear of
counter attacks in their bases, this takes their focus away from your
own bases, so power units can allow you to expand and tech more and
get more ahead.

Don't get the wrong idea, power units will not win you the game, nor
should you use them thinking that, there will be times where you will
surprise your opponent and win that y but the core principle for using
these units is to get further ahead.

power units are low on larva since you only need a couple of them to make your opponent think twice about attacking you, if he sees you got 1 infestor he might be afraid of walking in to a fungal growth and loosing his army and with mutalisks he will have to protect his mineral line so it doesn't get sniped while he attackes, and you only need to make your enemy think you got alot of them, you don't actually need allot of them so it gives you time to build other units
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BlemNov 17 '11 at 14:32

The power units in Zerg are Infestors, Mutalisks, Ultralisks and Broodlords.

Why is this particularly important for Zerg? Because unlike Protoss and Terran, which have dedicated production buildings, each unit made for a Zerg army means one less drone.

Take the scenario where a Zerg needs to kill 5 marines. Instead of making 12 zerglings, which costs 6 larvae (6 less drones), the Zerg can make 1 ultralisk and 5 drones (assuming resources aren't the limiting factor).

This actually relates to another interesting discussion in Theorycraft. When engaging a Zerg army, it's generally good strategy to target down the "power units", the ones that have high gas costs (infestors, mutas, ultras, broodlords), because gas is a limiting resource.

But some people argue that it can be useful to target units that have high larvae cost, mainly zerglings. Since zerglings need to used in large numbers to be effective, replacing an army of 60 zerglings indirectly hurts the Zerg player's potential to pump drones and scale the economy.